processing (OLTP), which is a low volume, latency sensitive application,
requires quick access to requested information. Backup processing application
require high bandwidth but are not sensitive to latency. In a network that does
not support service differentiation, all traffic is treated identically—they
experience similar latency and are allocated similar bandwidths. The QoS
feature in the Cisco MDS 9000 Series switches provides these guarantees.
Data traffic can be
prioritized in distinct levels of service differentiation: low, medium, or high
priority. You can apply QoS to ensure that Fibre Channel data traffic for your
latency-sensitive applications receive higher priority over
throughput-intensive applications such as data warehousing.
Figure 1. Prioritizing Data Traffic
In the above figure,
the OLTP traffic arriving at Switch 1 is marked with a high priority level of
throughput classification (class map) and marking (policy map). Similarly, the
backup traffic is marked with a low
priority level. The traffic is sent to the corresponding priority
queue within a virtual output queue (VOQ).
A deficit weighted
round robin (DWRR) scheduler configured in the first switch ensures that high
priority traffic is treated better than low priority traffic. For example, DWRR
weights of 70:20:10 implies that the high priority queue is serviced at 7 times
the rate of the low priority queue. This guarantees lower delays and higher
bandwidths to high priority traffic if congestion sets in. A similar
configuration in the second switch ensures the same traffic treatment in the
If the ISL is
congested when the OLTP server sends a request, the request is queued in the
high priority queue and is serviced almost immediately since the high priority
queue is not congested. The scheduler assigns its priority over the backup
traffic in the low priority queue.
When the high
priority queue does not have traffic flowing through, the low priority queue
uses all the bandwidth and is not restricted to the configured value.
A similar occurrence
in Switch 2 sends a response to the transaction request. The round trip delay
experienced by the OLTP server is independent of the volume of low priority
traffic or the ISL congestion. The backup traffic uses the available ISL
bandwidth when it is not used by the OLTP traffic.